1. Field of the Invention
The present invention relates to a still image transmitting method suitable for use in transmitting and receiving a video image signal of a wide band through a transmission line of a narrow band employing a bandwidth compression technique.
2. Description of Related Art
MUSE (Multiple Sub-Nyquist Sampling Encoding) is a system developed as a transmitting method for high-definition television (HDTV) broadcasting with the use of a satellite. The MUSE system is such that it enables a video image signal, whose resolution as visually sensed is kept from worsening by means of digital processing and bandwidth compression, to be transmitted with the use of a narrow band (8.1 MHz).
In the MUSE system, different signal processing is applied to the moving picture portion and to the still picture portion of the video image signal to be transmitted and the signal is transmitted at the rate of one for four sampling points.
One picture area of video image is formed of one field in the moving picture portion, whereas the same is formed of four fields in the still picture portion.
More specifically, in the case of the still video image transmission, first, the image data .largecircle. shown in FIG. 7(a), which is image data sub-sampled at the rate of one for four sampling points, is transmitted and, then, the data .DELTA. in the even field shown in FIG. 7(b) is transmitted as the second-field data.
Further, as shown in FIG. 7(c), the image data in the odd field is transmitted as the third-field data and, then, as shown in FIG. 7(d), the image data in the even field is transmitted as the fourth-field data.
The receiver side, upon receipt of the image data of four fields, synthesizes a first-field signal by generating an interpolated signal from the image data .largecircle. and and simultaneously generates a second field signal by obtaining an interpolated signal from the image data .DELTA. and .
Through such signal processing, the monitor of the received video image signal is enabled to display a monitor screen in which the image data of four fields are arranged as shown in FIG. 7(e).
In FIG. 7(e), the points .sup.. correspond to the image data generated through interpolation between fields. In the case of a still video image, there is no possibility of afterimage to be caused by the interpolation between fields.
However, the interpolated data .sup.. are contiguous in the diagonal direction. Therefore, the resolution in the diagonal direction is lowered to 1/2 approximately, and thus there has been a problem that the worsening of resolution becomes noticeable in a still video image.